Got my sign off today, W00T!
As far as middle-aged SAs, well, I'm middle-aged, and so is C_D, my tag out is older than me, and one of our ABUs, who has become a friend of mine, is older than me as well. Yes, most people are in their 20s or 30s, but there are plenty of people over 40 or older. If you fit the body type, you'll be cast, and remember, you can be cast for a mask role. My ABU friend was a zombie on my cast-wearing a mask, but also a short skirt. She happened to be the proper size for the costume.
When you audition, there is a space for availability on the audition sheet. Even if you don't fill out anything (and I will probably note 2 days to attend the event), you are given the chance to request days off when you first start rehearsals. Remember to request them right away, because days off requests are usually closed a week or two after the event starts, or at least that's how it was in my area. Remember that they understand the cast wanting to attend as well, so one or two days to do so isn't a big deal.